1. Field of the Invention
This invention relates to a method of controlling an induction motor power-fed by a power converter.
2. Prior Art
In recent years, as a method of power-feeding and controlling an induction motor by using a power converter, there has been developed a control method called a vector control to deal with current and/or magnetic flux, etc. of the induction motor as a vector quantity to control them. By such vector control, it has become possible to stably control an induction motor by a quick response like a d.c. motor. Thus, induction motors have been widely used as control motors.
Meanwhile, although there is a limitation in an output maximum voltage of a power converter for feeding a power to an induction motor, since the induced voltage becomes high when the rotational speed becomes high, the induction motor is brought into an uncontrollable state when the rotational speed is above a predetermined value. For this reason, there has been carried out a magnetic flux weakening control to weaken magnetic flux to thereby lower an induced voltage to permit control up to a higher speed rotational region. In order to weaken the magnetic flux, it is sufficient to allow the magnetic flux component current of the induction motor to be small. However, since the secondary winding of the induction motor is generally in a short-circuited state, damper action takes place, so a current to cancel a change in the magnetic flux component current transiently flows in the secondary winding. For this reason, the magnetic flux cannot vary immediately. A method of solving such a problem is disclosed in JP-B 57-38116 (the Japanese Patent Publication No. 38116/1982). In the method disclosed in this literature, it is assumed that the inductance of the induction motor is fixed. However, in actual induction motors, the inductance may vary by magnetic saturation. In speed control of general induction motors, there is not a serious problem in practical use although such magnetic saturation is not taken into consideration. Meanwhile, in recent years, such uses of induction motors to control the torque are increasing. In such cases of controlling the torque of the induction motor, if the influence of the magnetic saturation is neglected, it is impossible to attain quick-responding, stable and high accuracy control.